The solar cell is a semiconductor device for converting light energy to electricity and includes p-n junction type, pin type and Schottky type, with the p-n junction type being on widespread use. When classified in terms of substrate material, the solar cell is generally classified into three categories, crystalline silicon solar cells, amorphous silicon solar cells, and compound semiconductor solar cells. The crystalline silicon solar cells are sub-divided into monocrystalline and polycrystalline solar cells. Since crystalline silicon substrates for solar cells can be relatively easily manufactured, the crystalline silicon solar cells are currently manufactured at the largest scale and will find further widespread use in the future. See JP-A H08-073297 (Patent Document 1), for example.
In general, output characteristics of a solar cell are evaluated by measuring an output current-voltage curve by means of a solar simulator. On the curve, the point where the product of output current Imax by output voltage Vmax, Imax×Vmax, becomes the maximum is designated maximum power point Pmax. The conversion efficiency η of the solar cell is defined as the maximum power point Pmax divided by the overall light energy (S×I) incident on the solar cell:η={Pmax/(S×I)}×100(%)wherein S is a cell area and I is the intensity of irradiated light.
For increasing the conversion efficiency η, it is important to increase short-circuit current Isc (output current value at V=0 on the current-voltage curve) or Voc (output voltage value at I=0 on the current-voltage curve) and to make the profile of output current-voltage curve as close to squareness as possible. It is noted that the degree of squareness of an output current-voltage curve is generally evaluated by the fill factor (FF) which is defined as:FF=Pmax/(Isc×Voc).As the value of FF is closer to unity (1), the output current-voltage curve approaches ideal squareness, indicating an increase of conversion efficiency η.
For increasing the conversion efficiency η, it is important to reduce the surface recombination of carriers. In the crystalline silicon solar cell, minority carriers photo-generated by incidence of sunlight reach the p-n junction mainly via diffusion before they are externally extracted as majority carriers from electrodes attached to the light-receiving surface and back surface to provide electric energy.
At this point, those carriers which may be otherwise withdrawn as current flow can be lost by recombination via the interfacial level available on the substrate surface other than the electrode surface, leading to a lowering of conversion efficiency η.
Thus, in high-efficiency solar cells, an attempt to improve conversion efficiency η is by protecting the light-receiving and back surfaces of a silicon substrate with insulating films except for areas in contact with electrodes, for thereby restraining carrier recombination at the interface between the silicon substrate and the insulating film. As the insulating film, a silicon nitride film is useful and used in practice. This is because the silicon nitride film has the function of an antireflective film for crystalline silicon solar cells and is fully effective for the passivation of the surface and interior of the silicon substrate.
In the prior art, the silicon nitride film is formed by chemical vapor deposition (CVD) processes such as thermal CVD, plasma-enhanced CVD, and catalytic CVD. Of these, the plasma-enhanced CVD is the most widespread process. FIG. 1 schematically illustrates a parallel plate type plasma-enhanced CVD apparatus which is generally known as direct plasma CVD. The CVD apparatus 10 in FIG. 1 includes a vacuum chamber 10c defining a deposition compartment 1. Disposed in the deposition compartment 1 are a tray 3 for resting a semiconductor substrate 2 in place, a heater block 4 for maintaining the tray 3 at a predetermined temperature, and a temperature controller 5 for controlling the temperature of the heater block 4. The deposition compartment 1 is also provided with a deposition gas inlet line 6 for introducing preselected deposition gas as reactant gas into the deposition compartment 1, a radio-frequency power supply 7 for supplying energy to the introduced gas to generate a plasma, and a pumping unit 8.
When an insulating film is deposited in the illustrated CVD apparatus, the preselected deposition gas is introduced into the deposition compartment 1 at the predetermined flow rate through the gas inlet line 6, and the radio-frequency power supply 7 is operated to create a radio-frequency electric field. This operation generates a radio-frequency discharge to excite the deposition gas into a plasma, whereupon an insulating film is deposited on the surface of semiconductor substrate 2 via plasma-induced reaction. For example, when a silicon nitride film is deposited, a mixture of silane and ammonia gases is introduced as the deposition gas into the deposition compartment 1 through the gas inlet line 6, whereupon a silicon nitride film is deposited utilizing decomposition reaction of silane in plasma.
The plasma-enhanced CVD process is often used in forming an insulating film for solar cells since a high deposition rate is achievable even when the process temperature is as low as about 400° C. However, since high-energy charged particles created in the plasma tend to cause damage to the film being deposited and the silicon substrate surface, the resulting silicon nitride film has a higher interfacial level density, failing to exert a satisfactory passivation effect. Thus, for improving the passivation effect, it is necessary to block a dangling bond with hydrogen or the like.
To address the above problem, for example, JP-A 2005-217220 (Patent Document 2) proposes a remote plasma-enhanced CVD process as the method capable of suppressing plasma damage. FIG. 2 schematically illustrates one exemplary apparatus. The remote plasma-enhanced CVD apparatus shown in FIG. 2 includes a cylindrical excitation compartment 93 for exciting a reactant gas introduced therein into plasma, and a reaction compartment (or treating compartment) 98 disposed below the excitation compartment 93 in fluid communication therewith. The excitation compartment 93 is provided at its top with an inlet port 93a for a carrier gas 91, and at its center with a radio-frequency introducing portion (or waveguide) 93c which is connected to a microwave power source 95 via a matching unit 94. A supply line for a reactant gas 97 for deposition is connected to the reaction compartment 98, and a substrate holder 99 for holding a substrate 99a is disposed in the reaction compartment 98. With the apparatus of such construction, first microwave is introduced into the excitation compartment 93 from the microwave power source 95 to excite the carrier gas 91, the excited carrier gas 91 is introduced into the reaction compartment 98 in accordance with a gas pumping stream, and the reactant gas 97 is introduced in the reaction compartment 98 where it is activated and contacted with the substrate 99a, whereby a film is formed on the substrate 99a. Using ammonia gas as the carrier gas 91 and silane gas as the reactant gas 97, for example, a silicon nitride film can be formed on the substrate 99a. Since the remote plasma-enhanced CVD apparatus is constructed such that the substrate is placed at a position remote from the plasma region 96, the plasma damage to the substrate may be mitigated to some extent.
Also, JP-A 2009-117569 (Patent Document 3) reports that the passivation effect is improved when plasma treatment using ammonia gas is carried out as pretreatment, prior to the deposition of a silicon nitride film by surface wave plasma. JP-A 2009-130041 (Patent Document 4) reports that the passivation effect is improved when treatment with a plasma generated using a gas mixture of hydrogen gas and ammonia gas is carried out, prior to the deposition of a silicon nitride film.
However, since the above-cited methods need an extra process separate from the insulating film forming process, there arise the problems of an increased production cost and difficulty to improve productivity.
Further, if the composition of a silicon nitride film formed by the plasma-enhanced CVD is shifted from the stoichiometry to a silicon rich side so as to form a positive fixed charge, band bending occurs. Near the contact interface between silicon substrate and silicon nitride film, an inversion layer in which electrons are rich on the silicon substrate side is formed. Utilizing this, the passivation effect on the n-type region side can be enhanced.
JP-A 2002-270879 (Patent Document 5) reports that conversion efficiency is improved by a two-layer structure which is constructed by forming a silicon nitride layer having a high refractive index as a first dielectric film, and then forming a silicon nitride layer having a low refractive index thereon as a second dielectric film. This method, however, needs separate processes for forming high and low refractive index silicon nitride layers. For example, a silicon nitride layer having a high refractive index is first formed, after which the flow rate of deposition gas, after which a ratio of flow rates of ammonia gas and silane gas is adjusted, and then a silicon nitride layer having a low refractive index is formed. The method results in an increase of production cost and is difficult to improve productivity.